1. Field of the Invention
This invention relates in general to ion implantation and in particular to an operationally positional source magnetic field for an ion implantation system.
2. Description of the Related Art
Ion implantation systems are utilized for implanting ions in a substance. For example, ion implantation systems can be used to implant impurity ions in a semiconductor substrate of a wafer to form doped regions.
FIG. 1 is a diagram of an example of an ion implantation system for implanting ions into a target semiconductor wafer. Ion implantation system 101 includes an ion source system 105, an analyzing magnet 107, resolving aperture 109, and an acceleration tube 111. Ion implantation system 101 also includes a scanning system to distribute ions uniformly over a target wafer 131. In FIG. 1, the scanning system includes focus lens 112, neutral beam trap and beam gate 108, Y axis scanner 113, beam trap and gate plate 115, and X-axis scanner 119. Ion implantation system 101 also includes a system end station 132. End station 132 includes an area defining aperture 135, a Faraday cup and current integrator (which directly measures the implant dose by collecting the beam current and integrating over the implant time), and a subsystem that loads, holds, and positions the target wafer 131 for ion implementation. The Faraday cup nearly surrounds target wafer 131 and has an opening around aperture 135. Ion implantation system 101 also includes a vacuum system 133 for evacuating various chambers and components of the ion system. Ion implantation system 101 further includes a system controller 143 for controlling the operation of the ion implantation system. A further description of an example of an ion implantation system can be found in the Silicon Processing for the VLSI Era, by S. Wolf and R. N. Tauber, Vol. 1, 1986, pages 308-317. Other examples of ion implantation systems can be found in Plumb et al, U.S. Pat. No. 4,743,767; Aitken U.S. Pat. No. 5,300,785; Tanaka et al., U.S. Pat. No. 5,306,921; Mekenna et al., U.S. Pat. No. 4,757,208; Aitken et al., U.S. Pat. No. 5,389,793; Wong, U.S. Pat. No. 5,675,152; all of which are incorporated by reference in their entirety.
FIG. 2 is an illustrative diagram of an example of an ion source system of an ion implantation system. Ion source system 201 is a Bernas type ion source system. Located within arc chamber 203 is pig tail type filament 205 and repeller plate 207. A current provided through filament 205 generates thermal electrons which are accelerated through a gas species to collide with atoms of the gas species to produce ions. The gas species material is provided from a feed source (not shown) via the gas feed inlet 211. Ions exit the chamber through extraction slit 227.
The ion source system shown includes two source magnet poles that generate a magnetic field (designated as the dashed arrows) in arc chamber 203. The magnetic field alters the electron paths (e.g., path 222) to increase the probability of collisions with source material gas species atoms, thereby producing more ions. Source magnet poles for conventional ion implantation systems are operationally fixed with respect to the arc chamber such that they are capable of providing a magnetic field in only one position with respect to the arc chamber. Some ion implantation systems include the ability to vary the magnetic field provided by the source magnet poles 217 and 219. Varying the magnetic field allows the field strength to be set to a value that provides the maximum ion output for the source magnet core poles as positioned.
Because the magnetic field generated by a source magnet affects the ionization in the arc chamber, the magnetic field may create xe2x80x9chot spotsxe2x80x9d of ion plasma in the arc chamber which can shorten the life of the filament in the chamber.
It has been discovered that providing an ion implantation system with source magnetic structures that are operationally positionable with respect to an arc chamber advantageously provides the ion implantation system with the ability to move a magnetic field to maximize ion generation efficiency. Moving a magnetic field with respect to the arc chamber may also allow potential hot spots in the ion chamber to be moved within the chamber, thereby extending the operating life of the filament.
In one aspect of the invention, a method for generating ions includes generating ions from an arc chamber with source magnet poles providing a magnetic field in a first position with respect to the arc chamber. The method also includes positioning the source magnet poles to provide a magnetic field in a second position. The second position is different from the first position. The method further includes generating ions from the arc chamber with the magnetic field in the second position.
In another aspect, the invention includes an ion source system for an ion implantation system. The ion source system includes an arc chamber and a source magnet assembly including at least two magnetic poles that are positionable with respect to the arc chamber to provide a plurality of operational magnetic field positions with respect to the arc chamber.
In another aspect of the invention, an ion implantation system includes a ion source system including an arc chamber. The ion implantation system also includes means for providing a magnetic field for the ion source system in a plurality of operational positions with respect to the arc chamber.